1. Field of the Invention
The present invention relates to a process for producing a bipolar semiconductor device, particularly a process comprising steps of simultaneously driving impurities into an emitter region and gold atoms into a collector layer.
2. Description of the Prior Art
It is necessary to improve the switching speed of semiconductor devices for use in logic circuits, i.e., the minority carriers of holes in the collector layer must disappear rapidly in its off-state operation. In order to decrease the life time of the minority carriers, gold is diffused into the collector layer in the silicon wafer.
There are two types of diffusion of gold atoms into the silicon crystal lattice, i.e., interstitial type and substitutional type, and only the gold atoms which are substituted for silicon atoms serve to promote the combination of holes and electrons, i.e., the switching speed of the semiconductor devices. The interstitial gold atoms diffuse among the silicon crystal lattice relatively rapidly. On the other hand, the substitutional gold atoms diffuse very slowly by replacing silicon atoms.
It is necessary not to convert the substitutional gold atoms into the interstitial gold atoms by rapidly cooling the silicon wafer.
In the prior art, gold is applied on the bottom surface of the silicon wafer either by evaporating gold or by spin coating a gold-containing solution and thereafter gold is driven forward into the collector layer located near the top surface of the wafer by heat treatment. This process is successfully carried out when the thickness of a silicon wafer is as thin as about 200.about.300 .mu.m.
However, when the diameter of a silicon wafer is increased to about 90 mm, it becomes necessary to increase its thickness to 700 .mu.m in order to avoid its deformation during heat treatment. The inevitable elongation of the heating time causes higher structural stress in the semiconductor devices located on the surface of the silicon wafer and deteriorates their electrical characteristics. The diffusion of gold atoms from the bottom surface through the silicon wafer is a long distance, and the interstitial gold atoms diffuse relatively rapidly in the silicon lattice up to the collector layer, where the silicon atoms are not yet substituted by the gold atoms.